Comparing Hysteretic Behavior of RC Frames Retrofitted with Low-Yield-Point (LYP) Steel Core BRB and Perforated Steel Plate Shear Wall (PSPSW)

This paper aims to compare effective retrofitting methods for existing reinforced concrete (RC) buildings using updated response control techniques. The study presents results of a near full-scale experimental study of sub-standard RC frames retrofitted with both steel core buckling restrained braces (BRBs) and thin perforated steel plate shear walls (PSPSWs). Older public buildings with higher importance factors like schools and hospitals do not satisfy the up-to-date code requirements generally lack strength, stiffness, and ductility and typically require retrofitting to enhance their seismic performance. BRBs functioning as metallic hysteretic dampers are new generation bracing systems developed both for existing or new buildings and bridges, and provide an increase in structural integrity and reduce seismic demands through stable energy dissipation until core fracture. Closed steel frames (SF) designed to remain elastic within the target limits are preferred to provide an efficient/distributed load transfer between the BRBs and RC frame. The proposed solution helps preserve the lateral stiffness while enhancing self-centering capability of the frame especially at inelastic cycles. As an alternative method and for comparison, this paper also investigates a possible retrofit concept, which features a combination of PSPSW and closed steel frames. In the experimental program, the RSB specimen which is an RC frame retrofitted by BRB with the core material having a low yield point steel (LYP225) is investigated firstly. The BRB is attached to the SF with welded end connections at the gusset plates. Steel frame is connected to the RC frame by using a specially designed joint consisting of chemical anchors on RC frame, shear studs on SF, and ladder stirrups for controlling cracks in the grout. The joint between the SF and RC frame is finally filled with high strength grout to provide a strong composite action for the full transfer of loads caused by the yielding of the BRB core. RSP specimen, which implements PSPSW as retrofit member, used the same joint detail with RSB specimen for a better comparison. In the RSP specimen, PSPSW with circular perforations having a diameter of D=300mm spaced at 400mm diagonal distance were designed to develop an appropriate tension field action in the specimen. Both specimens showed stable hysteretic behavior without fracture up to 1/150 story drift (0.67%), designated as the target retrofit drift for damage controlled design. Behavioral values such as load-displacement hysteretic curves, effective damping ratios, and total dissipated hysteretic energies are calculated and compared for both specimens. Dissipated energies per volumetric steel material used and inelastic demands placed on the RC frame for both specimens are also given for a better comparison. These tests show that PSPSWs and BRBs increase ductility to an adequate seismic performance level while controlling damage at a minimum.